How to Choose a Portable Air Conditioner: BTU, Noise and Venting Explained

Portable air conditioners are the easiest cooling appliance to buy badly. They all look similar, they are all advertised with a big number on the box, and the specification that actually decides whether you will be happy — how loud the thing is at night — is often missing from the listing altogether. Here is what to look at, in the order that matters.

1. Start with the room, not the appliance

Cooling capacity is measured in BTU per hour. As a rough working figure, a reasonably insulated room needs somewhere around 20 BTU per square foot (roughly 200 BTU per square metre). A 15 m² bedroom therefore lands in the 7,000–9,000 BTU range.

Adjust upwards if any of these apply:

• Large or south-facing windows without blinds — add 10–20%.
• Top floor or attic room sitting under a hot roof — add 10–20%.
• Kitchen, or a room with a lot of electronics — add roughly 1,000 BTU per regular occupant beyond the first, and more for an oven or a gaming PC.

Oversizing is not free, either. A unit that is far too powerful cools the air quickly, switches off, and never runs long enough to pull humidity out. The room ends up cold and clammy.

2. Single hose or double hose

Every portable unit has to send hot air outside through a window kit. A single-hose model takes the air it expels from inside the room, which quietly pulls warm outdoor air in through every gap in the building to replace it. A dual-hose model draws its exhaust air from outside instead, so it does not depressurise the room.

Dual-hose units are bulkier and usually cost more, but in a larger or leakier room the difference in real-world performance is noticeable. In a small, well-sealed bedroom, a single hose is normally fine.

3. Noise is the specification people regret ignoring

Anything above roughly 55 dB is difficult to sleep next to. A good portable unit sits in the 48–54 dB range on its lowest setting; a compact one that has to work hard can hit 60 dB and above. If a listing does not publish a decibel figure at all, treat that as information in itself, and go looking for video reviews where you can hear the unit running.

4. Drainage

Air conditioning removes water from the air, and that water has to go somewhere. Three common designs:

1. Self-evaporating — most of the condensate is blown out with the exhaust. The most convenient option, but in very humid weather you will still have to empty a reservoir.
2. Manual tank — you empty it. Fine occasionally, tedious daily.
3. Continuous drain — a hose runs to a floor drain or out of the window. Best if you have somewhere for it to go.

5. Installation reality check

Before ordering, measure your window. Standard kits fit sliding and sash windows easily. Casement windows — the kind that swing outwards on a hinge, common across much of Europe — often need a different kit or a custom acrylic panel. Check that the hose is long enough to reach the window from wherever the unit can physically stand, and remember that a long, unsupported hose radiates heat back into the room.

6. Running cost

Multiply the unit's power draw in kilowatts by your electricity price and by the hours you expect to run it. A 1 kW unit running eight hours a night at €0.25/kWh costs about €2 a night — roughly €60 over a hot month. Not enormous, but worth knowing before rather than after.

A short buying checklist

• Room area measured, BTU figure calculated, with a margin for sun and occupancy
• The lower of the two BTU ratings compared across models
• A published noise figure, ideally under 55 dB on low
• A window kit that actually fits your window type
• A drainage arrangement you can live with
• A stated returns window and warranty from the seller

Get those six right and the rest — remote controls, timers, app connectivity — is preference rather than performance.